ELEVATED IMPOSED WORK OF BREATHING MASQUERADING AS VENTILATOR WEANINGINTOLERANCE

Objective: To test the hypothesis that, if apparent ventilatory insuff iciency observed during a weaning or preextubation trial is due to a s ignificant contribution of imposed work of the endotracheal tube and b reathing apparatus (WOBImp), and the patient's actual physiologic work of breathing (WOBPhys) is not excessive, it should be possible to ext ubate these patients safely. Design: Prospective descriptive study. Pa tients: A total of 28 (17% of all ventilated patients) adults intubate d for 48 h or longer, who developed tachypnea (40+/-9 breaths/min) but whose blood gas exchange met predefined extubation criteria, were eva luated over a 3-month period. Interventions: Using a microprocessor-ba sed monitor (Bicore Monitoring Systems Inc, Irvine, Calif) total patie nt work of breathing (WOBTOT) was determined by integrating the change in intraesophageal pressure with tidal volume measured with a miniatu re pneumotachograph positioned at the airway opening. If the patient's WOBTOT was equal to or greater than 0.8 J/L, WOBImp was determined by integrating the changes in carinal pressures with tidal volume. If ne ither the patient's WOBTOT or WOBPhys was excessively greater than tha t of spontaneous breathing at rest tie, <0.8 J/L: normal range, 0.5 to 0.6 J/L), the patient was extubated. Measurements and results: Breath ing frequency, peak inspiratory flow rate (PIFR), auto-Peep (PEEPa), d ynamic compliance (CDXN) WOBTOT, WOBImp, resistance to expiratory airw ay flow (RAWE) were measured, and WOBPhys calculated (WOBTOT minus WOB Imp). The means and SDs were calculated, and data were analyzed by unp aired t test and linear regression. Six patients (5%) were found to ha ve WOBTOT Of <0.8 J/L and were successfully extubated without determin ation of WOBImp. Twenty-one patients were found to have an elevated WO BTOT (1.6+/-0.83 J/L), and had WOBImp measured. In these 21 patients, WOBImp (1.1+/-0.04 J/L) was twice WOBPhys (0.5+/-0.26 J/L). Extubation was successful in 20 of 21 patients in which WOBPhys was determined n ot to be excessive tie, <0.8 J/L). The last patient had an elevated WO BPhys (1.4 J/L) and was not extubated until his disease improved later . Overall, reintubation rate was 4%. Conclusion: Increased WOBTOT may be misinterpreted as a patient failure tie, tachypnea) and weaning hal ted or extubation not done, prolonging intubation. The ability to meas ure the contribution of WOBImp to WOBTOT can identify those patients w ho may be safely extubated when WOBPhys (WOBTOT minus WOBImp) is accep table and the apparent ventilatory insuffiency is related to significa nt WOBImp.